Conventional microlithographic systems project images of masks (also referred to as reticles) onto photosensitive substrates. For projecting modified or even larger patterns, the masks must be replaced with other masks containing the different or extended patterns.
So-called “maskless” or “reticle-free” microlithographic systems replace the masks with spatial light modulating arrays and projection systems for generating controllable patterns onto substrates. The spatial light modulating arrays regulate transmissions of individual transverse segments of light through the microlithographic systems for projecting patterns of spots onto photosensitive substrates. Each spot is regulated by one or more addressable elements of the light modulating arrays. Under programmed control, the patterns of spots vary with a relative translation of the photosensitive substrates to expose a continuously adjustable pattern of spots on the substrates.
The addressable elements of the spatial light modulating arrays function as micromechanical switches for controlling whether or not the individual transverse segments of the light reach the photosensitive substrates. Different optical mechanisms can be used for the switching function such as phase shifting as provided by grating light valve devices or directionally controlled reflections as provided by digital micromirror devices.
Typically, the projection systems form a magnified image of each of the individually addressable elements on corresponding microlenses of a microlens array, and the individual microlenses of the microlens array concentrate the light from the individual addressable elements through focused spots. Together, the magnification and focusing functions result in the focused spots being spaced apart. The pattern of focused spots includes multiple rows of focused spots and is oriented at a slight angle to a direction of translation with respect to the substrate so that successive rows of the focused spots provide for selectively illuminating the entire scanned area of substrate.
Highly resolved spots are needed for patterning closely spaced features on the substrates in sharp relief. Increased resolution is achieved by the multistage projection system in which the light segments emerging from the addressable elements are directed through respective foci, and the foci are relayed to form the regulated spots on the substrate. However, imperfections within the addressable elements can distort the size and shape of the spots, and thereby lessen resolution of the resulting projected image.
For example, the micromechanical mirrors of digital micromirror devices can include surface irregularities that depart from idealized flat specular surfaces of the micromirrors. The departures in slope increase the range of angular reflections of light from the micromirrors resulting in a corresponding increase one or more dimensions of the focused spots. While it may be possible to stop down an aperture of an imaging lens within the projection system for removing the increased range of angular reflections of light from the micromirrors, the reduced aperture size also limits the capability of the projection system to distinguish between light segments emerging from adjacent micromirrors. Such so-called “crosstalk” between adjacent micromechanical mirrors reduces contrast of the projected patterns by overlapping images of the micromirrors on the microlens array. Instead of each microlens receiving light from just its associated micromirror, inadequately imaged light from individual micromirrors can spread into adjacent microlenses and partially illuminate focused spots not intended for generating the desired image.